Our preliminary studies have identified two original cardioprotective effects of histamine H3-receptor activation. These are: 1) The mimicking of a new paradigm of ischemic preconditioning, which prevents the activation of a local mast cell-dependent renin- angiotensin system (RAS) and its dysfunctional consequences; and, 2) The inhibition of a previously unsuspected pro-adrenergic effect of cardiac natriuretic peptides. An in- depth exploration of these novel H3-receptor-mediated cardioprotective effects is the main goal of this application. Aim I seeks to define: a) how ischemic preconditioning prevents the activation of the mast cell-dependent RAS, thus alleviating norepinephrine- and angiotensin-induced arrhythmias; and, b) how H3-receptor activation mimics the cardioprotective anti-RAS effects of preconditioning. The contribution of adenosine A2b/A3-receptors, protein kinase C5 (PKC5) and aldehyde dehydrogenase type-2 (ALDH2) to preconditioning-mediated anti-RAS effects will be studied in isolated guinea- pig and PKC5-/- mouse hearts subjected to ischemia/reperfusion, and in cultured mast cell, both wild-type and depleted of PKC5 and ALDH2 by siRNA technology. The capacity of H3-receptors to inhibit the release of mast cell-degranulating neuropeptides from in isolated hearts. Roles of PKC2I, 7 and 8 in inhibiting mast cell degranulation will be studied in mast cell cultures. Aim II seeks to determine by which mechanisms natriuretic peptides exert a pro- adrenergic effect and how this is inhibited by H3-receptor activation. We will assess whether the catecholamine-releasing effects of natriuretic peptides derive from a cGMP/PKG-mediated prevention of cAMP hydrolysis by PDE3 and whether H3-receptor activation limits these proadrenergic effects by inhibiting PKG and/or stimulating PDE3. Isolated hearts, sympathetic nerve endings and PC12 cells, both wild-type and PKG- and PDE3-depleted by siRNA, will be used. Collectively, these studies will elucidate new mechanisms for the control of renin and norepinephrine release in the heart. As the search for effective cardioprotective drugs continues unabated, our proposed studies will foster the design of new agents (e.g., selective H3-receptor agonists) mimicking the beneficial effects of preconditioning and enabling a safe and effective treatment of congestive heart failure with natriuretic peptides. and ATP sensory/sympathetic nerves will be assessed